Method of locating a subsea structure for deployment

ABSTRACT

A method of locating a subsea structure beneath a floating deployment vessel for deployment to a seabed including at least the steps of: (a) transporting the subsea structure on a floating transporting vessel near to the deployment vessel; (b) lowering the transporting vessel to allow the subsea structure to float; (c) relocating either the transporting vessel or the subsea structure to allow the subsea structure to be separate of the transporting vessel; and (d) locating the subsea structure beneath the deployment vessel using one or more buoyancy elements with variable buoyancy.

The present invention relates to a method for locating a subseastructure for deployment, generally for subsequent deployment to aseabed.

The term “subsea structure” refers to any equipment, tool, machine,package, unit, device or other item or installation to be located and/orinstalled on or near a seabed, including but not limited to risers,underwater well-head elements in oil fields, manifolds, protectionstructures, fluid separation, pumping or processing units. Many of suchstructures are usually relatively ‘large’ and/or ‘heavy’.

Subsea structures that are intended to be installed on the seabed areusually manufactured onshore. Especially larger subsea structures aretransported to a deployment site by towing them through the water behinda suitable vessel. At the deployment site, they are directly loweredfrom a deployment vessel to an installation site on the seabed. WO03/074353 A1 shows an example of a method for transporting andinstalling objects at sea being part of an infrastructure in oil and gasfields offshore. However, towing a large subsea structure all the wayfrom its onshore manufacture has difficulties.

Subsea structures of lesser size or weight can be transported on thedeck of a suitable vessel or transportation barge to the deploymentsite, at which the subsea structure is then lifted off the vessel andinto the sea for subsequent deployment. However, this requires thepresence of a crane with usually heavy lifting capacity.

PI 0306058-6A discloses a pendular method for installing equipment atthe bottom of the sea, whereby the equipment can be transported on thedeck of a first craft, and then swung like a pendulum until it is nearthe seabed. However, this results in an increased tension on the cable,and potentially a lack of control in the deployment, whilst not reducingin any way the load on the deployment winch.

It is an object of the present invention to provide an improved methodof locating a subsea structure for deployment to the seabed.

Thus, according to a first aspect of the present invention, there isprovided a method of locating a subsea structure beneath a floatingdeployment vessel for deployment to a seabed comprising at least thesteps of:

-   -   (a) transporting the subsea structure on a floating transporting        vessel near to the deployment vessel;    -   (b) lowering the transporting vessel to allow the subsea        structure to float;    -   (c) relocating either the transporting vessel or the subsea        structure to allow the subsea structure to be separate of the        transporting vessel; and    -   (d) locating the subsea structure beneath the deployment vessel        using one or more buoyancy elements with variable buoyancy.

In this way, the subsea structure is safely transported to its site ofdeployment on a suitable floating vessel; it can be launched into thesea at a ‘splashzone’ without requiring a crane or other liftingmechanism; and then its location beneath the deployment vessel forsubsequent deployment to the seabed can be controlled by the use of theone or more suitable buoyancy elements. The present invention thereforeprovides a controlled passage through the splashzone without therequirement for a crane, and can reduce the load on the deployment winchon a deployment vessel by modifying the net submerged weight of thesubsea structure with the buoyancy element(s).

The floating transporting vessel may be any suitable self-propelled shipable to move between a normal sea-faring/travelling/shipping positionand a semi-submersible/submerged position. Such vessels are known in theart, for example from Dockwise Shipping B.V. Such vessels generally havean open-deck, and are able to be heavy transport sea and ocean-goingvessels. A subsea structure can be located either directly on or next toa suitable deck of the floating transporting vessel, optionally on orwithin or in association with a suitable frame or cradle. The subseastructure can be ‘sea-fastened’ to the deck of such a vessel fortransportation, and the fastenings for which can be released prior tothe commencement of deployment.

The floating transporting vessel transports the subsea structure near tothe deployment vessel. This can include transport next to or otherwisesufficiently within the vicinity of the deployment vessel, preferablysuch that any umbilicals or cables or like required to be connectedbetween the subsea structure and the deployment vessel can be connectedat sea level.

Once ready to launch at a ‘splashzone’, usually at or near thedeployment site and near or next to the deployment vessel, the floatingtransporting vessel is able to semi-submerge, generally by the use ofone or more buoyancy and/or ballast tanks therein, so as to still befloating, but to have at least a part of the transporting vessel,usually an open-deck, underwater.

Once the transporting vessel is in its semi-submerged position, thesubsea structure can float or be floated at or near sea level, either byitself or by being launched. The transporting vessel may include one ormore assistant means such as cranes or winches to assist movement of thesubsea structure in relation to the semi-submerged transporting vessel,especially once the subsea structure is able to float, but any suchassistant means are not intended to be able to freely lift the subseastructure in air.

The subsea structure may be associated with one or more buoyancy aids,one or more of which may be fitted onshore, at the deployment siteand/or both, to assist its ability to float once the transporting vesselhas been lowered to a semi-submerged position.

Various buoyancy aids are known in the art. Generally it is preferred touse only one type or form of buoyancy aid to launch and deploy a subseastructure, due to the complexity of making any mechanical changes duringsuch operations.

Buoyancy aids in the form of one or more air tanks attached to a subseastructure are well known in the art, and are advantageous to reduce theapparent weight of the structure by increasing its buoyancy in water.Such buoyancy aids are used conventionally to reduce the load borne byuse of a conventional crane to deploy a subsea structure, but they donot reduce the ‘hook load’ during lift of the subsea structure from thedeck of a vessel, through the splashzone, and through deployment. Suchtanks must also be strong enough to be capable of withstanding maximumexternal hydrostatic pressure without imploding or deforming due to thecompressibility of the air contained therein, thus increasing the weightof the tanks and thereby reducing their buoyancy.

In one embodiment of the present invention, the buoyancy aid comprisesthe one or more buoyancy elements with variable buoyancy of step (d).

Preferably, the method of the present invention comprises the furtherstep of providing at least one buoyancy element with variable buoyancyon or connected to the subsea structure during step (b). Optionally, oneor more of such buoyancy elements are on or connected to the subseastructure onshore, and are transported to the splashdown and/ordeployment site with the subsea structure by the transporting vessel.

In the present invention, a buoyancy element with variable buoyancy maycomprise any arrangement. Preferably, such buoyancy elements compriseone or more sections, parts, tanks and/or chambers able to contain atleast a proportion of a buoyancy fluid comprising a wholly orsubstantially incompressible fluid having a density less than that ofsea water. One or more chambers could be formed from a flexible orelastic material to permit the volume thereof to vary and accommodatethe volume of buoyancy fluid contained therein.

Varying the amount of buoyancy fluid in the buoyancy element(s) variesthe overall buoyancy of the associated structure in a manner known inthe art.

An external reservoir of buoyancy fluid can be provided to vary thevolume of buoyancy fluid within the or each buoyancy element, having theeffect of varying the volume of said one or more chambers, etc.

Such buoyancy elements can also contain air, and optionally also water,at varying times and varying proportions. For example, the or eachchamber or tank, etc. could contain a bladder or diaphragm to allow suchchamber(s) to simultaneously comprise buoyancy fluid and water, and alsoto allow the proportions of the buoyancy fluid and water in suchchamber(s) to be varied, without allowing the two components to mix.

Prior to submerging or submersion of the subsea structure, all chambersin the buoyancy element(s) are preferably fully vented of air andreplaced with either the low-density incompressible buoyancy fluidand/or water.

The substantially incompressible nature of the buoyancy fluid means thatthe buoyancy element can be made form relatively thin and light weightmaterial compared to prior art air tanks, thus reducing the weight ofthe buoyancy elements and increasing the buoyancy thereof.

Suitable buoyancy fluids include low molecular weight hydrocarbons suchas methanol. In order to further reduce the density of the buoyancyfluid, glass microspheres may be added. Such fluids preferably have adensity of between 500 and 600 kg/m³ (the density of seawater beingaround 1027 kg/m³). However any fluid having a density less than that ofseawater may be suitable as the buoyancy fluid. Preferably the buoyancyfluid also has a low viscosity such that it is easily moveable (such asby one or more pumps) between the buoyancy element and an externalreservoir, and such fluids include known low viscosity gels and thelike.

The use of one or more buoyancy aids, such as a buoyancy elementdescribed above, in association with the subsea structure, may assistthe floating of the subsea structure at or near sea level following thelowering of the transporting vessel to its semi-submerged position.Following the floating of the subsea structure, there is relocation ofeither the floating vessel or the subsea structure (or both) to allowthe subsea structure to be separate of the floating vessel. This mayinvolve movement of the semi-submerged vessel, but usually involves therelocation of the floating subsea structure away from the transportingvessel such that the subsea structure is then floating ‘free’ of thelocation of the transporting vessel.

Following the relocation of step (c), the transporting vessel can befully re-floated in a manner known in the art, generally by the emptyingof one or more of its ballast tanks.

The launch or splashdown of the subsea structure may be separate orremote from the deployment site, and the subsea structure can then betowed the remaining distance to the deployment site. For example, it maybe desired to launch the subsea structure in calmer or more shelteredseas, or in or at a calmer or more sheltered site, than occur at thedeployment site. Thus, some relocation of the launched and nowsemi-submersed subsea structure may occur between steps (c) and (d) ofthe present invention.

The subsea structure, and/or any associated buoyancy aids such as abuoyancy element, may include one or more guidelines or control lines toassist or control movement of the subsea structure in the sea, either asit floats at or near sea level, or during its subsequent movement, orboth. Where the launch or splashdown of the subsea structure is separatefrom the deployment site, one guideline may be to partly, substantiallyor fully assist towing of the subsea structure to the deployment site.

In one embodiment of the present invention, the subsea structurecomprises a fastening line. During transportation of the subseastructure to the splashdown and/or deployment site, the fastening linemay not be in use, or may be used to assist securing the subseastructure to the transporting vessel.

The method of the present invention further comprises the step ofsecuring a fastening line associated with the subsea structure to thedeployment vessel at the deployment site. The securing of a fasteningline to the deployment vessel may occur at any stage prior to, during orafter step (b) and/or step (c), preferably prior to step (b), of themethod of the present invention.

Prior to step (d) of the method of the present invention, one or morebuoyancy elements with variable buoyancy are associated with subseastructure. Such association may be by securement or attachment or otherconnection of the one or more buoyancy elements to the subsea structure,generally in or at a position above the subsea structure. Preferably, atleast one such buoyancy element is associated with the subsea structureonshore, and remains associated with the subsea structure during steps(b) and (c) for use in step (d). The association between the buoyancyelement(s) and the subsea structure is preferably designed or adapted toaccommodate both tensile and compressive forces between the components,which can or may occur during the different phases of the deployment andlanding operation. Preferably, the association is also designed oradapted to be remotely released by a Remotely Operated Vehicle (ROV)after the subsea structure is landed on a seabed or other intendedworking location to allow controlled recovery of the buoyancyelement(s).

A reservoir of buoyancy fluid for the or each buoyancy element may belocated at any suitable location, including on one or more floatingvessels such as independent storage vessels. By way of example only, onesuitable location of a reservoir of buoyancy fluid is on the deploymentvessel.

Fluid communication is required between any such reservoir and the oreach buoyancy element, and suitable fluid umbilicals for such fluidcommunication are well known in the art.

Step (d) of the method of the present invention comprises locating thesubsea structure beneath a deployment vessel, usually at the deploymentsite, using one or more buoyancy elements as described above. After step(c), and after any relocation to the deployment site, the subseastructure is floating at or near sea level near to the deploymentvessel, and the or each buoyancy element provides control of thepositioning of the subsea structure beneath the deployment vessel.

The term “beneath a deployment vessel” as used herein includes thesubsea structure being directly beneath the deployment vessel, generallyby the connection through one or more generally vertical channels orports in the deployment vessel, as well as next to or near to a side ofthe deployment vessel, optionally by the use of one more overhangingpulleys or winches or the like. The present invention is not limited bythe exact position of the subsea structure beneath the deployment vesselin preparation for deployment by the deployment vessel, conventionallyin a vertically downward direction, of the subsea structure to theseabed.

PI 0306058-6A shows lowering of equipment from sea level to near thepoint of installation by the swinging pendulum movement of the equipmentdownwardly to the seabed. Such equipment undergoes sideward ‘freefall’as it travels through the sea and thus creates increased strain on theinstallation cable.

By the use of one or more buoyancy elements with variable buoyancy, thepresent invention is able to provide controlled location of the subseastructure from a floating sea level position, to a suitable positionready for deployment beneath the deployment vessel.

In particular, by transferring buoyancy fluid in/out of the one or morebuoyancy elements to vary the volume of buoyancy fluid in the or eachbuoyancy element, very close control of the overall buoyancy of thesubsea structure can be provided so as to control the relocation of thesubsea structure beneath the deployment vessel. In this way, careful andcontrolled lowering of the subsea structure from the sea level positionto a position beneath the deployment vessel, can be carried out withoutany ‘freefall’, especially pendular freefall, and consequent strain onthe fastening line therebetween.

Varying the buoyancy of the or each buoyancy element associated with thesubsea structure may involve the transferring of one or more of theelements: air, water (generally and preferably seawater) and buoyancyfluids; in, out, within, between, or any combination thereof; the one ormore of the buoyancy elements. Means such as pumps, valves, lines,inlets and outlets of the or each buoyancy element are known in the art,and the skilled man is able to provide control of the presence and/orflow of the or each fluid so as to vary the buoyancy of the or eachbuoyancy element to provide controlled lowering or decent of the subseastructure from its floating sea level position to beneath the deploymentvessel.

Naturally, the subsea structure has a certain weight, such that the oreach buoyancy element requires a certain degree of buoyancy to allow thesubsea structure to float even at sea level. This can involve theinclusion of a proportion of air and/or buoyancy fluid in one or morechambers, sections, tanks, etc of the or each buoyancy element. Somewater may also be included to prevent over-buoyancy.

Preferably, prior to submersion of the subsea structure, all air isevacuated from the buoyancy element(s) and replaced by either waterand/or buoyancy fluid, thus removing the requirement for the buoyancyelement to be designed to accommodate a net external hydrostaticpressure. The transfer of buoyancy fluid and/or water to replace the airalso reduces the overall buoyancy of the or each buoyancy element,providing careful control of the rate (and optionally position) of thelowering the subsea structure such that it can be slowly and carefullylocated beneath the deployment vessel.

In a particular embodiment of the present invention, step (d) comprisesa method of lowering a subsea structure to beneath the deployment vesselcomprising the steps of:

providing at least one buoyancy element on or connected to the subseastructure, said at least one buoyancy element comprising one or morechambers containing a buoyancy fluid comprising a substantiallyincompressible fluid having a density less than that of sea water;

-   -   providing a reservoir for said buoyancy fluid at a location        remote from said subsea structure;    -   providing fluid communication between said reservoir and said        one or more chambers of said at least one buoyancy element;    -   transferring said buoyancy fluid between said reservoir and said        one or more chambers of said at least one buoyancy element to        vary the volume of buoyancy fluid within the at least one        buoyancy element and thus vary the overall buoyancy of the        subsea structure to thereby initiate and subsequently control        the rate of descent of the subsea structure.

Subsea structures can be deployed to an installation site, generally onthe seabed, using a number of known methods. Generally, these involvethe lowering of the subsea structure by means of a winch and fasteningline from a floating deployment vessel directly beneath the vessel andin a controlled manner.

The subsequent deployment of a subsea structure used in the presentinvention to the seabed may optionally but preferably continue toinvolve one or more of the buoyancy elements used in step (d),preferably by or involving at least partly the replacement of buoyancyfluid by water in a controlled manner to allow the subsequent loweringof the subsea structure beneath the deployment vessel to the seabed.

Thus, according to a second aspect of the present invention, there isprovided a method of deploying a subsea structure to a seabed comprisingthe steps of:

-   -   (a) transporting the subsea structure on a floating transporting        vessel near to a deployment vessel;    -   (b) lowering the transportation vessel to allow the subsea        structure to float;    -   (c) relocating either the transporting vessel or the subsea        structure to allow the subsea structure to be separate of the        transporting vessel;    -   (d) locating the subsea structure beneath the deployment vessel        using one or more buoyancy elements with variable buoyancy; and    -   (e) deploying the subsea structure from beneath the deployment        vessel to the seabed.

Preferably, step (e) comprises replacing buoyancy fluid in one or moreof the buoyancy elements with water.

Embodiments of the present invention will now be described by way ofexample only, and with reference to the accompanying diagrammaticdrawings in which:

FIG. 1 shows a first stage of a method of locating a subsea structureaccording to an embodiment of the present invention;

FIG. 2 shows a second stage of the method;

FIG. 3 show a third stage of the method;

FIG. 4 shows the location of a subsea structure beneath the deploymentvessel;

FIGS. 5 and 6 show the deployment of the subsea structure to the seabedaccording to another embodiment of the present invention;

FIGS. 7 to 9 show the recovery of a buoyancy element from the seabed.

The present invention provides an improved method for locating a subseastructure, especially a large subsea structure for use in oil and gasfields offshore, beneath a deployment vessel, and for subsequentdeployment of the subsea structure to the seabed, such as to a deepwater seabed location.

FIG. 1 shows the subsea structure 10 located on the open-deck 12 of afloating and sea-faring or sea-going transporting vessel 14, such as theknown marine vessel “Sea Serpent”. The transporting vessel 14 is able totransport the subsea structure 10 from a dock or other onshore location(not shown) to near the deployment vessel 26 already at a deploymentsire, or possibly at a more suitable, such as more sheltered, waterlocation for the splashdown.

FIG. 1 shows the option of including a buoyancy element 16 attached tothe subsea structure 10. The attachment may occur onshore, or after thesubsea structure 10 has been located in the floating vessel 14. Thebuoyancy element 16 could be in the form of a ‘variable buoyancysubmersible barge’ (VBSB).

The buoyancy element 16 may include one or more chambers (not shown) andone or more fluid umbilicals 18 able to provide fluid communication fortransferring buoyancy fluid between the buoyancy element 16 and areservoir once required. During transporting of the subsea structure 10as shown in FIG. 1, the fluid umbilical 18 may not be required.

FIG. 1 also shows a support guide 20 above the buoyancy element 16,attached to a fastening line 22. The fastening line 22 may not berequired during transporting of the subsea structure shown in FIG. 1.The support guide 20 provides even support of the buoyancy element 16and subsea structure 10 via the fastening line 22.

Once near the deployment vessel 26, and possibly at or near thedeployment site, FIG. 2 shows the lowering of the transporting vessel 14to a semi-submerged position, generally by the flooding of one or moreballast tanks therein in a manner known in the art. Preferably prior tothe lowering of the transporting vessel 14, the fastening line 22 isconnected to a suitable winch 24 on the nearby deployment vessel 26. Thefastening line 22 may pass through a vertical channel in the deploymentvessel 26 to reach the winch 24.

Similarly, the fluid umbilical 18 is connected to a suitable reservoir30 on the deployment vessel 26. Meanwhile, any fastenings or othersecurement of the subsea structure 10 to the transporting vessel 14 canbe detached and/or released.

As the transporting vessel 14 semi-submerges, the subsea structure 10starts to float at or near the sea level 15, generally because of thebuoyancy provided by the buoyancy element 16. The skilled man will beaware of the amount of buoyancy required in the buoyancy element 16 toachieve floating of the subsea structure 10, and generally at least aportion, optionally substantially or all, of the buoyancy element 16,may be air-filled at this time.

By use of the fastening line 22 and/or one or more other guide orcontrol elements not shown, the floating subsea structure 10 can berelocated gently away from the transporting vessel 14, whilst stillfloating at or near sea level. The energy required to relocate thefloating subsea structure 10 is clearly substantially less than thatrequired to vertically lift any subsea structure in the air by a craneover the side of a transporting vessel and into the sea.

Once the subsea structure 10 and buoyancy element 16 are ‘free’ of thetransporting vessel 14, generally floating in a different sea area thanthe location of the transporting vessel 14, the transporting vessel 14can be re-floated to its normal floating position, for re-use.

FIG. 3 shows the floating subsea structure 10 and buoyancy element 16next to the deployment vessel 26 following the removal of thetransporting vessel 14. By controlled transfer of:

(a) buoyancy fluid in the reservoir 30 into the buoyancy element 16,optionally into one or more parts, sections, tanks or chambers therein,and distributed therein in an equitable manner; and/or

(b) water, generally being sea water, through one or more inlets (notshown) into the buoyancy element 16 in a manner known in the art;

the flooding of the buoyancy element (and exhaust of air through one ormore outlets (not shown), such outlets optionally being cone-topped toreduce and/or prevent the entrapment of air where not desired), thebuoyancy of the buoyancy element 16 is reduced in a controlled manner toallow the lowering of the subsea structure 10 from its floating sealevel position gently to a position beneath the deployment vessel 26along a path 28 shown in FIG. 3 by four arrows.

The buoyancy element 16 may include one or more sections, chambers ortanks able to be ‘hybrid’ tanks to accept the presence of buoyancy fluidand water, generally in a pre-determined and/or variable volume and/orratio.

The use of buoyancy fluid having a density less than that of waterand/or the use of water itself, allows the skilled man to very carefullycontrol the buoyancy of the buoyancy element 16 to counter the weight ofthe subsea structure 10 and allow it to gently sink, preferably untilthe overall combination of the buoyancy element 16 and subsea structure10 achieves neutral buoyancy at a position beneath the deployment vessel26 as shown in FIG. 4.

Preferably, the submerged weight of the subsea structure 10 is reduced(by the use of the buoyancy element 16 containing a combination of waterand sufficient low density incompressible buoyancy fluid), so as to be amaximum of 75% of the safe working capacity of the deployment winch 30.However, whilst the submerged weight of the subsea structure 10 has beenreduced, the mass of the subsea structure 10 has not been reduced andneither has its inertia. The winch 30 may therefore still comprise or befitted with specific features, known in the art, to help control thesize of dynamic peak loads expected in the fastening line 22 induced bythe motions of the deployment vessel 26.

FIGS. 3 and 4 also show the use of an ROV (Remotely Operated Vehicle) 34from the deployment vessel 26 to monitor and/or assist any and allmovements of the subsea structure 10 and the buoyancy element 16.

Once the subsea structure 10 is beneath the deployment vessel 26 asshown in FIG. 4, the subsea structure 10 is ready for deployment to thesea bed (such as to an installation site) as shown in FIGS. 5 and 6.

Deployment of a subsea structure 10 from a deployment vessel 26 may becarried out by various methods known in the art, generally involvingdecreasing the buoyancy of one or more attachments to the subseastructure 10.

In an embodiment of the present invention, further transfer of buoyancyfluid out of the buoyancy element 16 back to the reservoir 30, andreplacement therewith by water, provides negative buoyancy to thebuoyancy element 16 such that the subsea structure 10 is able to fall tothe seabed 36 in a controlled manner.

The removal of the buoyancy element 16 from the subsea structure 10 oncelocated on the seabed 36, and the recovery of the buoyancy element 16 tothe sea surface 15, are shown in FIGS. 7 to 9. A towing line or pennant38 can be attached to the buoyancy element 16, and the reverse of someof the steps mentioned hereinabove can occur to increase under controlthe buoyancy of the buoyancy element 16 to the sea surface 15.

Various modifications and variations to the described embodiments of theinvention will be apparent to those skilled in the art without departingfrom the scope of the invention as defined herein. Although theinvention has been described in connection with specific preferredembodiments it should be understood that the invention as defined hereinshould not be unduly limited to such specific embodiments.

For example, in one alternative arrangement, the launch or splashdown ofthe subsea structure 10 shown in FIG. 2 occurs in a sheltered water siteseparate from the deployment site, and the subsea structure 10 andassociated buoyancy element 16 are towed by the deployment vessel 26 tothe deployment site prior to locating the subsea structure 10 beneaththe deployment vessel 26 as shown by the path 28 in FIG. 3 leading toFIG. 4.

1. A method of locating a subsea structure beneath a floating deploymentvessel for deployment to a seabed comprising at least the steps of: (a)transporting the subsea structure on a floating transporting vessel nearto the deployment vessel; (b) lowering the transporting vessel to allowthe subsea structure to float; (c) relocating either the transportingvessel or the subsea structure to allow the subsea structure to beseparate of the transporting vessel; and (d) locating the subseastructure beneath the deployment vessel using one or more buoyancyelements with variable buoyancy.
 2. A method as claimed in claim 1wherein the floating transporting vessel is moveable in step (b) betweena sea-faring position and a semi-submerged position.
 3. A method asclaimed in claim 1 wherein the subsea structure is located in step (a)directly on or next to a deck of the floating transporting vessel.
 4. Amethod as claimed in claim 1 further comprising the step of providing atleast one buoyancy element with variable buoyancy on or connected to thesubsea structure prior to or during step (b).
 5. A method as claimed inclaim 4 wherein the at least one buoyancy element is on or connected tothe subsea structure onshore and/or prior to step (a).
 6. A method asclaimed in claim 4 wherein the at least one buoyancy element isconnected to the subsea structure onshore, and remains associated withthe subsea structure during steps (b) and (c) for use in step (d).
 7. Amethod as claimed in claim 1 wherein the one or more buoyancy elementswith variable buoyancy are secured to the subsea structure in or at aposition above the subsea structure.
 8. A method as claimed in claim 1wherein each buoyancy element comprises one or more chambers able tocontain at least a proportion of a buoyancy fluid.
 9. A method asclaimed in claim 8 wherein one or more of the chambers are formed from aflexible or elastic material to permit the volume thereof to vary andaccommodate the volume of buoyancy fluid contained therein.
 10. A methodas claimed in claim 8 wherein the buoyancy fluid comprises a wholly orsubstantially incompressible fluid having a density less than that ofsea water.
 11. A method as claimed in claim 10 wherein the buoyancyfluid is a low molecular weight hydrocarbon such as methanol.
 12. Amethod as claimed in claim 8 wherein the buoyancy fluid includes glassmicrospheres.
 13. A method as claimed in claim 8 wherein the buoyancyfluid has a density of between 500 and 600 kg/m³.
 14. A method asclaimed in claim 8 wherein the buoyancy fluid is a low viscosity gel.15. A method as claimed in claim 8 further comprising the step ofvarying the volume of buoyancy fluid within the or each buoyancy elementto vary the overall buoyancy of the subsea structure.
 16. A method asclaimed in claim 8 further comprising providing an external reservoir ofbuoyancy fluid and transferring buoyancy fluid between the externalreservoir and the or each buoyancy element.
 17. A method as claimed inclaim 8 wherein the or each chamber includes a bladder or diaphragm toallow the chamber(s) to simultaneously comprise buoyancy fluid andwater, preferably to allow the proportions of the buoyancy fluid andwater in such chamber(s) to be varied, without allowing the twocomponents to mix.
 18. A method as claimed in claim 1 comprising varyingthe buoyancy of the or each buoyancy element associated with the subseastructure by transferring of one or more of the elements: air, water,preferably seawater, and buoyancy fluids; in, out, within, between orany combination thereof; the one or more of the buoyancy elements.
 19. Amethod as claimed in claim 1 wherein prior to step (b), the buoyancyelement(s) are fully vented of air and replaced with either buoyancyfluid and/or water.
 20. A method as claimed in claim 1 comprising thefurther transporting of the floating subsea structure at or nearsea-level between steps (c) and (d).
 21. A method as claimed in claim 1including the steps of: providing at least one buoyancy element on orconnected to the subsea structure, said at least one buoyancy elementcomprising one or more chambers containing a buoyancy fluid comprising asubstantially incompressible fluid having a density less than that ofsea water; providing a reservoir for said buoyancy fluid at a locationremote from said subsea structure; providing fluid communication betweensaid reservoir and said one or more chambers of said at least onebuoyancy element; transferring said buoyancy fluid between saidreservoir and said one or more chambers of said at least one buoyancyelement to vary the volume of buoyancy fluid within the at least onebuoyancy element and thus vary the overall buoyancy of the subseastructure to control the rate of descent of the subsea structure tobeneath the deployment vessel.
 22. A method as claimed any claim 1wherein the subsea structure is subsequently deployed to an installationsite on the seabed beneath the floating deployment vessel.
 23. A methodas claimed in claim 1 wherein the subsequent deployment of a subseastructure to the seabed involves the one or more of the buoyancyelements used in step (d), preferably by or involving at least partlythe replacement of buoyancy fluid in the buoyancy element(s) by water.24. A method of deploying a subsea structure to a seabed comprising thesteps of: (a) transporting the subsea structure on a floatingtransporting vessel near to a deployment vessel; (b) lowering thetransportation vessel to allow the subsea structure to float; (c)relocating either the transporting vessel or the subsea structure toallow the subsea structure to be separate of the transporting vessel;(d) locating the subsea structure beneath the deployment vessel usingone or more buoyancy elements with variable buoyancy; and (e) deployingthe subsea structure from beneath the deployment vessel to the seabed.25. A method as claimed in claim 24 wherein step (e) comprises replacingbuoyancy fluid in one or more of the buoyancy elements with water.